It's going to be released this summer
SP2 is said to have many new features (besides many hotfixes that Micro$oft made XP have), such as Download Manager for IE (which was introduced in Longhorn). I personally want to see the Concurrent Login Session (i.e. User A is connected through Remote Desktop, and User B is a console user and they can use the computer at the same time. Currently only one user can "use" the computer at the same time while others' work/processes are saved in memory).
Well, the currently leaked build is 2055 I think.